Experimentation is directed toward elucidating selected aspects of the three dimensional structure of cytochrome Cl as it exists in the inner mitochondrial membrane and to describe the changes in structure which this membrane protein undergoes as it participates in the oxidation-reduction reactions of the electron transport chain. This investigation is part of a long range goal of identifying each of the components of the cytochrome B-Cl complex and assigning to each of these a structural and/or functional role in the electron transfer and energy conserving activities of this segment of the inner mitochondrial membrane.